Many differences in behavior have been noted between males and females in addition to the behaviors associated with reproduction. One of these sexually dimorphic behaviors is the pup-killing response: male rats tend to kill young pups whereas females do not. In seeking the developmental determinants of this behavior it seemed reasonable to regard pup-killing behavior as a sexual characteristic and as a behavior likely to be under the influence of the sex hormones. Research generated from this working hypothesis has shown that pup killing is very definitely related to the presence of testosterone during early development. The experiments in the present proposal are designed to explore the ontogenetic contingencies between the development of pup-killing behavior and the hormonal factors which regulate the development of sexual characteristics. The hormone-behavior relationships which emerge should increase our fundamental understanding of the role of the endocrine system in the differentiation of sex-typical behavior.